Lung cancer Is the leading cause of cancer death in the U.S. and worldwide. The Hsp90 chaperone is required for the stability of multiple oncogenic kinases that drive signaling, proliferation and survival of nonsmall cell lung cancers (NSCLCs), including mutant EGFR, Her2, B-Raf, c-Met and cdk4. Inhibitors of HspQO will be studied including geldanamcyins such as 17-AAG, the water-soluble derivatives IPI-504 and 17-DMAG, and STA-11-9090, a novel non-geldanamycin, to compare their relative potencies, to define pharmacodynamic endpoints and to conduct clinical trials in molecularly defined patient subgroups. In the first specific aim, these compounds will be studied in EGFR mutant NSCLC cells, including those expressing mutant EGFR harboring the T790M secondary mutation conferring erlotinib resistance. The ability of Hsp90 inhibitors to deplete mutant EGFR and to suppress downstream signaling of the PI3K-Akt-mTOR-p70S6K pathway will be assessed. The relative potencies of 17-DMAG and STA-11 -9090 will be compared to 17- AAG in isogenic cell line models in vitro, and in EGFR mutant/T790M NCI-H1975 xenografts in vivo. The activity of these compounds will also be evaluated in mutant EGFR-driven models of lung adenocarcinoma. In the second specific aim, the activity of Hsp90 inhibitors will be assessed in EGFR wild-type cells driven by other Hsp90 clients. Additionally, Hsp90 inhibitor-mediated depletion of IGF-1R will be evaluated in cells expressing EGFR: IGF-1R heterodimers to determine if there is cytotoxic synergy with erlotinib. In the third specific aim, synerglsm of 17-AAG with other agents that disrupt chaperone function or Hsp70 induction will be explored, including inhibitors of HDAC6, the proteasome or cyclin-dependent kinase 9. In the fourth specific aim, a Phase l/ll Trial of IPI-504 will be conducted;after establishment of the maximum tolerated dose (MTD), preliminary antitumor activity will be defined in NSCLC patients harboring either EGFR mutant or wild-type tumors. A Phase I trial of STA-11-9090 will also be performed to establish the MTD .and safety profile, Hsp90 client depletion will be evaluated in tumor biopsy specimens and peripheral blood nononuclear cells. In summary, survival for advanced NSCLC remains poor. Many proteins that drive lung cancer growth depend on a chaperone called Hsp90 for their stability and function. This work will explore compounds that inhibit Hsp90 in preclinical models and clinical trials as potential treatments for lung cancer.